This invention relates to a control system and method for an engine, and more particularly to a fuel control for an engine that operates with a feedback control principle during at least a portion of its operation.
In the interest of providing good fuel economy and effective exhaust emission control, it has been proposed to employ a feedback control system for controlling the air-fuel ratio of the engine. The feedback control acts primarily on the fuel supply to maintain the desired air-fuel ratio, although other systems are possible.
When operating with feedback control systems, it is the practice to use a sensor that provides a signal indicative of the actual air-fuel ratio supplied to the engine. Oxygen (O.sub.2) sensors are commonly utilized for this purpose. The oxygen sensor senses the amount of oxygen in the exhaust gases, and from this it can determine whether the actual mixture supplied to the combustion chamber is lean or rich, or at the desired ratio.
In order to provide an accurate indication of the actual combustion chamber conditions, it is desirable if the sensor is positioned as close as possible to the exhaust discharge of the engine. That is, the sensor should be as close as possible to the point where the exhaust gases exit the combustion chamber.
With multiple cylinder engines, therefore, it is difficult to utilize only a single sensor for all cylinders of the engine. If the sensor is positioned downstream of the exhaust ports of the cylinders far enough to receive an average signal, it will be too far from the cylinders to be accurate and helpful. Therefore, many systems provide an arrangement wherein only one sensor is utilized to control the fuel supply amount for all of the cylinders.
Where this is done, however, it is the normal practice to assume that each cylinder is operating on substantially the same fuel-air ratio as the other. That is, when an adjustment is made for the sensed cylinder, the same adjustment is made for the remaining cylinders. With many engines and engine configurations, however, this assumption is not only not accurate, but can be at substantial variance from the actual engine conditions.
For example, in outboard motor practice, the exhaust system is relatively compact and, as a result, the difference in the distance from each exhaust port to the end of the exhaust discharge is substantially different. This provides varying in-cylinder conditions which effect the optimum air-fuel ratio for each cylinder.
It is, therefore, a principal object of this invention to provide an improved feedback control system for a multiple cylinder engine that employs only a single sensor for all cylinders.
It is a further object of this invention to provide a feedback control system of this type wherein the sensor is positioned so that the condition in all cylinders can be adjusted accurately based upon this one sensing position.
In connection with the use of such an arrangement utilizing only a single cylinder, it may be desirable to provide fixed differences in the amount of feedback control adjustment applied to each cylinder. However, the variation in adjustment also is not uniform under all conditions.
It is, therefore, a still further object of this invention to provide an improved feedback control system for a multi-cylinder engine utilizing only a single sensor and wherein the adjustments to the nonsensed cylinders are made depending upon running conditions.
In many systems that employ feedback control, there are times when the feedback control is not utilized, but an open control is utilized. For example, the type of sensor previously referred to (an O.sub.2 sensor) must be at a certain operating temperature before it will give a reading. As a result, it has been proposed to employ an open control for the system during initial start-up and until the oxygen sensor reaches its operating temperature. Thus, when the shift-over from open control to feedback control occurs, there will be, more than likely, some adjustment that needs to be made.
In addition, during feedback control operation, adjustments also are required in order to bring the air-fuel ratio to the desired condition. Generally, these adjustments are made in incremental steps, and the incremental adjustment is the same, regardless of whether being made during the switch-over condition or during transient conditions of feedback control.
Another condition when there may be a shifting between feedback control and open control is if the sensor is deemed to be somehow inoperative or providing unreliable signals after it has reached its operating temperature. It has been proposed to provide an arrangement for detecting such conditions and switching over to open control when this occurs. Again, however, the adjustment during such shiftover positions is made on the same basis as during transient conditions, and this also may not be desirable.
It is, therefore, a still further object of this invention to provide an improved control system and method for an engine that is operable on both open and feedback controls and wherein the incremental adjustments are varied depending upon the condition which exists. This reduces the likelihood of hunting and permits more rapid return to the desired air-fuel ratio independently of the cause for the deviation.